Just one week after fertilization, the embryo signals the mother of her pregnancy by producing human chorionic gonadotropin (hCG). When the ovaries detect hCG, they continue producing progesterone, which maintains the uterine lining for implantation.
The embryonic stage transitions to the fetal stage about 10 weeks into pregnancy (8 weeks after fertilization). Key milestones include:
Heart starts beating: Week 5
Most organs formed: By Week 12
Gender detectable by ultrasound: Around Week 14
Mother feels fetal movement: Between Weeks 16–20
Modern medicine continues to uncover amazing insights about fetal development during this critical stage.
1. How the Fetal Heart Works
The heart begins beating just over 3 weeks after fertilization, even before it is fully formed.
By week 6.5, the heart has four chambers and beats over 160 times per minute.
By week 7.5, its electrical activity resembles an adult heart.
The fetal heart pumps blood without using the lungs and even contributes stem cells to the mother’s heart for repair.
Early heartbeats are autonomous, not controlled by the brain, and are much faster than the mother’s heart rate—nearly double at times.
2. The Fetal Brain: A Marvel of Complexity
Brain development starts in week 3, forming the neural tube, which will become the brain and spinal cord.
Early months: Formation of major brain regions:
Forebrain: Thought and emotion
Midbrain: Movement and senses
Hindbrain: Breathing and balance
From month 4: Neural connections form; in the third trimester, synapses multiply, coordinating movement and responses.
The fetal brain reacts to stimuli in utero:
Light, touch, sound
Practices “breathing” motions via heart rate and diaphragm
Fetuses may even experience REM-like sleep, possibly dreaming, and recognize their mother’s voice from week 25.
3. Reflexive Movements
By week 6, reflexive movements appear:
Early head-turning: Response to mouth area touch
Week 8: Ability to grasp, move jaw, blink, or stretch toes
Floating in amniotic fluid: Allows rolling over, which newborns take months to master outside the womb
Other notable reflexes:
Startle reflex: Sudden arm extension when startled
Thumb-sucking: Prepares for feeding
Hiccups: Diaphragm exercise, healthy sign
Leg kicks with sudden stops: Primitive defensive reaction
Response to sound: Turns toward voice or music
Hand grasping: Reflex to touch against uterine wall or umbilical cord
Spinning movements: Development of balance and spatial awareness
Sleep movements: Twitches during sleep, similar to adult REM
4. Gender Differentiation
Female ovaries visible by week 7, uterus by week 9
External genitalia start forming weeks 9–11, but remain difficult to distinguish
Ultrasound guesses may appear around week 14, with more accuracy at weeks 16–18
5. Fetal Intestinal Movement
Intestinal development begins weeks 8–10 with small peristaltic movements
By weeks 12–16, meconium forms: a dark, sticky substance made of dead cells, amniotic fluid, and digestive secretions
Meconium is not expelled until birth
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